1. Field of the Invention
The present invention pertains to the art of thermoforming appliance liners and, more particularly, to a method and apparatus for developing a substantially uniform temperature across a plastic sheet for delivery to an appliance liner thermoforming device.
2. Discussion of the Prior Art
In forming liners for appliances, such as refrigerators and dishwashers, it is known to extrude sheets or billets of thermoplastic material to be delivered to a thermoforming device. Following the extrusion process, the sheets will be at a temperature in the order of 130° F. and are typically stacked for later delivery to the thermoforming device. At times, the sheets can remain in a given stack for a week or more prior to actually being formed into a liner.
The temperature environment in which the sheets are stacked is typically not uniformly regulated. For instance, the sheets can be placed in a 60° F. or less environment in the winter season and subjected to over 100° F. temperatures in the summer. In addition, since the sheets can remain in the stacks for some time, the outer stacked sheets will generally reach the ambient temperature, while the inner sheets will stay fairly hot. Furthermore, the outer edge portions of each inner sheet will assume a lower temperature than the center of the sheet.
Based on at least these reasons, the actual temperatures of the various sheets delivered to the thermoforming device can vary greatly which, in turn, can result in the production of thermoformed articles having varying wall thicknesses and varying amounts of “formed in” stresses. This problem is particularly significant in the field of the present invention wherein an extruded sheet of approximately 5 mm (0.20 inches) thick is stretched to form a liner in the order of two feet deep, with the liner walls being extremely thin, generally in the order of less than 0.7 mm (0.03 inches). With varying temperatures across the sheet, the sheet will unevenly stretch, perhaps resulting in a liner having certain wall portions which are disadvantageously thin and prone to cracking.
In an attempt to compensate for potential product variations, it has been proposed in the art to pre-heat plastic sheets to be thermoformed. For instance, U.S. Pat. No. 4,842,742 is directed to controlling the heating of different portions of a synthetic resin sheet delivered to a vacuum forming device to create a refrigerator liner. The actual purpose of the controlled heating is to provide a desired differential thickness in different portions of the liner. More specifically, an oven is provided to heat a sheet received from a stacked pallet prior to delivery of the sheet to a vacuum forming device. The heating oven includes upper and lower platens for the differential heating of the sheet. Other arrangements have also been proposed to maintain accurate temperature control in the heating of at least one side of a plastic sheet.
Regardless of the prior attempts made in pre-heating plastic sheets, there still exists a need for a method and apparatus for forming a liner of an appliance from an extruded sheet which is delivered at a substantially uniform temperature to a thermoforming device. More specifically, there exists a need for a compact and efficient device and method for providing a consistent and substantially uniform temperature sheet which is to be thermoformed into an appliance liner.